Field of the Invention
The present invention relates to a friction plate used in a clutch or a brake of an automatic transmission in vehicles, and a wet-type multiple-disc clutch provided with the friction plate.
Description of the Related Art
In general, a wet-type multiple-disc clutch in an automatic transmission has friction plates and separator plates, which are alternately disposed between the drum and the hub of the clutch or a brake, and the clutch is engaged and disengaged by pressing and releasing a clutch piston.
In such a wet-type multiple-disc clutch, there is an occurrence of drag torque leading to a power loss through a lubricant at the time of disengagement of the clutch. Hence, recently, reduction of the drag torque has become a major challenge for automatic transmissions to be compliant with the demand for environmental regulation and low fuel consumption in vehicles.
As a technique in the related art for reducing the drag torque in the friction plate of the wet-type multiple-disc clutch, for example, Japanese Patent Application Laid-Open No. 2005-076759 discloses that a forming groove is provided on a friction member surface of the friction plate, and Japanese Patent Application Laid-Open No. 2008-180314 discloses that a segment shape is adopted to a friction member of the friction plate, and oil is caused to pass through an oil channel between the friction members. Each of the effects (separating and the like) at the time of oil draining has been utilized so as to achieve an effect of reducing the drag torque at a certain variable rotation speed range of an automatic transmission.
While reduction of the drag torque has been highly demanded at the variable rotation speed range, particularly at a low rotation speed range, the countermeasures in the related art have problems described below.
When the forming groove is adopted onto the friction member surface of the friction plate, the groove brings a separating effect between the plates utilizing oil drainage, and the groove provides an effect of reducing the drag torque in the friction plate. However, if the area of the groove for draining oil is increased in order to acquire a better reduction effect, the friction area in the friction plate decreases, thereby resulting in a problem of degradation of heat resistance and durability.
When utilizing the shape of the segment in the friction member, it is possible not only to achieve the separating effect between the plates but also to significantly improve oil drainage and decrease viscous resistance (fluidity) after drainage. Thus, the drag torque is remarkably reduced. However, if the shape of the segment is utilized in order to improve the oil drainage in pursuing a better reduction effect, it is not possible for a lubricant to stably form an oil film on the surface of the friction member, thereby causing fluid shearing resistance hindering the reduction of the drag torque.
Meanwhile, Japanese Patent Application Laid-Open No. 2010-223342 has proposed that an air layer is formed on the surface by providing multiple fine grooves on the surface of the friction member, thereby reducing the drag torque. The disclosure merely presents an idea regarding an effect of reducing the drag torque under a lubricant environment, without mentioning a reduction effect at the variable rotation speed range.